In general, electronic power steering apparatuses (EPS) are provided with a motor serving as a drive source and a torque sensor. The torque sensor outputs a sensor signal based on torsion of a torsion bar provided in the middle of a steering shaft. Then, in response to a steering torque detected based on a sensor signal sent from the torque sensor, a torque in the motor is controlled so that an appropriate assist force is applied to a steering system in accordance with a steering torque. It is therefore necessary in electronic power steering apparatuses to detect a steering torque in a stable and highly accurate manner so that an appropriate assist force can be obtained.
For example, Patent Document 1 discloses a torque sensor for use in an electronic power steering apparatus for vehicle. The torque sensor disclosed in this document is provided with an input shaft, an output shaft, a torsion bar for connecting the input shaft and the output shaft, a ring-shaped magnet attached to the input shaft, a pair of magnetic yokes placed to surround the magnet, and magnetic sensors for detecting the density of a magnetic flux generated between the paired magnetic yokes. In this configuration, detection accuracy is improved by adding or averaging outputs from the plural magnetic sensors.
Another electronic power steering apparatus provided with a plurality of torque sensors is also disclosed in Patent Document 2. According to the electronic power steering apparatus disclosed in this document, even if an abnormal sensor signal is outputted from one of two torque sensors, an assist force can be applied continuously by using a sensor signal outputted from the remaining normal torque sensor.
Although Patent Document 1 teaches how to improve detection accuracy by increasing the number of magnetic sensors, a backup control is not taken into consideration in the event of an abnormal sensor signal generated by any one of plural systems and outputted from torque sensor. Patent Document 2 teaches, in the event of an abnormal sensor signal outputted from the remaining normal torque sensor, suspending assist provided by an assist device after passing of a predetermined period of time from a sudden change of a value detected by the torque sensors, but it takes no account of measures taken in a period from a sudden change of a detected value to suspension of assist. Thus, it is likely in conventional electronic power steering apparatuses that an assist force is applied to a steering system unintentionally during a period from detecting no normal sensor signal generated by plural systems and outputted from a torque sensor to suspending an assist force provided by a steering force assist device.